Membrane for blood purification

ABSTRACT

The present disclosure relates to semipermeable membranes which are suitable for blood purification, e.g. by hemodialysis, which have an increased ability to remove larger molecules while at the same time effectively retaining albumin. The membranes are characterized by a molecular retention onset (MWRO) of between 9.0 kD and 14.5 kD and a molecular weight cut-off (MWCO) of between 55 kD and 130 kD as determined by dextran sieving curves and can be prepared by industrially feasible processes excluding a treatment with salt before drying. The invention therefore also relates to a process for the production of the membranes and to their use in medical applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patent application Ser. No. 16/851,432, filed on Apr. 17, 2020, which is a continuation application of U.S. patent application Ser. No. 15/116,031, filed on Aug. 2, 2016, which is the U.S. national phase of PCT/EP2015/052364, filed on Feb. 5, 2015, which claims priority to European Patent Application 14154175.5, filed on Feb. 6, 2014. The disclosures of U.S. patent application Ser. No. 16/851,432, U.S. patent application Ser. No. 15/116,031, European Patent Application 14154175.5, and PCT/EP2015/052364 are hereby incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to semipermeable membranes which are suitable for blood purification, e.g. by hemodialysis, which have an increased ability to remove larger molecules while at the same time effectively retaining albumin. The invention also relates to a simplified process for the production of the membranes and to their use in medical applications.

DESCRIPTION OF THE RELATED ART

Dialysis membranes today are designed to accomplish the removal of uremic toxins and excess water from the blood of patients with chronic renal failure while balancing the electrolyte content in the blood with the dialysis fluid. Uremic toxins are usually classified according to their size (FIG. 1 ) and physicochemical characteristics in small water-soluble compounds (e.g., urea and creatinine), protein-bound solutes (e.g., p-cresyl sulfate) and middle molecules (e.g., b2-microglobulin and interleukin-6). While the removal of small molecules takes place mainly by diffusion due to concentration differences between the blood stream and the dialysis fluid flow, the removal of middle molecules is mainly achieved by convection through ultrafiltration. The degree of diffusion and convection depends on the treatment mode (hemodialysis, hemofiltration or hemodiafiltration) as well as on the currently available membrane type (low-flux high-flux, protein leaking, or high cut-off membranes).

The sieving property of a membrane, i.e., its permeability to solutes, is determined by the pore size and sets the maximum size for the solutes that can be dragged through the membrane with the fluid flow. The sieving coefficient for a given substance could be simply described as the ratio between the substance concentration in the filtrate and its concentration in the feed (i.e., the blood or plasma), and is therefore a value between 0 and 1. Assuming that the size of a solute is proportional to its molecular weight, a common way to illustrate the properties of membranes is by building a sieving curve, which depicts the sieving coefficient as a function of the molecular weight. The expression “molecular weight cut-off” or “MWCO” or “nominal molecular weight cut-off” as interchangeably used herein is a value for describing the retention capabilities of a membrane and refers to the molecular mass of a solute where the membranes have a rejection of 90%, corresponding to a sieving coefficient of 0.1. The MWCO can alternatively be described as the molecular mass of a solute, such as, for example, dextrans or proteins where the membranes allow passage of 10% of the molecules. The shape of the curve depends, to a significant extent, on the pore size distribution and to the physical form of appearance of the membrane and its pore structure, which can otherwise be only inadequately described. Sieving curves are thus a good description not only of the performance of a membrane but are also descriptive of the specific submacroscopic structure of the membrane.

In vitro characterization of blood purification membranes includes the determination of the removal rate for small and middle molecules as well as for albumin. For this purpose, filtration experiments are carried out with different marker solutes, among which dextran has been widely used since it is non-toxic, stable, inert and available in a wide range of molecular weights (Michaels A S. Analysis and Prediction of Sieving Curves for Ultrafiltration Membranes: A Universal Correlation? Sep. Sci Technol. 1980; 15(6):1305-1322. Leypoldt J K, Cheung A K. Characterization of molecular transport in artificial kidneys. Artif Organs. 1996; 20(5):381-389). Since dextrans are approximately linear chains, their size does not correspond to that of a protein with similar molecular weight. However, comparisons are possible once the radius of the dextran coiled chain is calculated. The sieving curve determined for a polydisperse dextran mixture can thus be considered a standard characterization technique for a membrane, and a number of recent publications have analyzed this methodology (Bakhshayeshi M, Kanani D M, Mehta A, et al. Dextran sieving test for characterization of virus filtration membranes. J Membr Sci. 2011; 379(1-2):239-248. Bakhshayeshi M, Zhou H, Olsen C, Yuan W, Zydney A L. Understanding dextran retention data for hollow fiber ultrafiltration membranes. J Membr Sci. 2011; 385-386(1):243-250. Hwang K J, Sz P Y. Effect of membrane pore size on the performance of cross-flow microfiltration of BSA/dextran mixtures. J Membr Sci. 2011; 378(1-2):272-279. 11. Peeva P D, Million N, Ulbricht M. Factors affecting the sieving behavior of anti-fouling thin-layer cross-linked hydrogel polyethersulfone composite ultrafiltration membranes. J Membr Sci. 2012; 390-391:99-112. Boschetti-de-Fierro A et al. Extended characterization of a new class of membranes for blood purification: The high cut-off membranes. Int J Artif Organs 2013; 36(7), 455-463).

Conventional dialysis membranes are classified as “low-flux” or “high-flux”, depending on their permeability. A third group, called protein leaking membranes, is also available on some markets. These three membrane groups were described in a review by Ward in 2005 (Ward R A. Protein-leaking membranes for hemodialysis: a new class of membranes in search of an application? J Am Soc Nephrol. 2005; 16(8):2421-2430). High-flux membranes used in devices, such as, for example, Polyflux® 170H (Gambro), Revaclear® (Gambro), Ultraflux® EMIC2 (Fresenius Medical Care), Optiflux® F180NR (Fresenius Medical Care) have been on the market for several years now. The high-flux membranes used therein are mainly polysulfone or polyethersulfone based membranes and methods for their production have been described, for example, in U.S. Pat. No. 5,891,338 and EP 2 113 298 A1. Another known membrane is used in the Phylther® HF 17G filter from Bellco Società unipersonale a r.l. It is generally referred to as high-flux membrane and is based on polyphenylene. In polysulfone or polyethersulfone based membranes, the polymer solution often comprises between 10 and 20 weight-% of polyethersulfone or polysulfone as hydrophobic polymer and 2 to 11 weight-% of a hydrophilic polymer, in most cases PVP, wherein said PVP generally consists of a low and a high molecular PVP component. The resulting high-flux type membranes generally consist of 80-99% by weight of said hydrophobic polymer and 1-20% by weight of said hydrophilic polymer. During production of the membrane the temperature of the spinneret generally is in the range of from 25-55° C. Polymer combinations, process parameters and performance data can otherwise be retrieved from the references mentioned or can be taken from publicly available data sheets. The expression “high-flux membrane(s)” as used herein refers to membranes having a MWRO between 5 kDa and 10 kDa and a MWCO between 25 kDa and 65 kDa, as determined by dextran sieving measurements according to Boschetti et al. (2013). The average pore radius is in the range of from 3.5 to 5.5 nm, wherein the pore size is determined from the MWCO based on dextran sieving coefficients according to Boschetti-de-Fierro et al. (2013) and Granath et al. (1967). Molecular weight distribution analysis by gel chromatography on sephadex. J Chromatogr A. 1967; 28(C):69-81. The main difference between high-flux membranes and low-flux membranes is a higher water permeability and the ability to remove small-to-middle molecules like β2-microglobulin.

Protein leaking membranes, on the other hand, have a water permeability similar to that of low-flux membranes, the ability to remove small-to-middle molecules similar to high-flux membranes, and they show albumin loss which is generally higher than that of high-flux membranes.

Lately a fourth type has emerged, called “high cut-off” membranes, which form a new group in addition to the ones mentioned before. This type of membrane has first been disclosed in WO 2004/056460 A1 wherein certain early high cut-off membranes are described which were primarily intended for the treatment of sepsis by eliminating sepsis-associated inflammatory mediators. Advanced dialyzers making use of high cut-off type membranes which are currently on the market are, for example, HCO1100®, septeX™ and Theralite®, all available from Gambro Lundia AB. Known uses of said advanced high cut-off membranes include treatment of sepsis (EP 2 281 625 A1), chronic inflammation (EP 2 161 072 A1), amyloidosis and rhabdomyolysis and treatment of anemia (US 2012/0305487 A1), the most explored therapy to date being the treatment of myeloma kidney patients (U.S. Pat. No. 7,875,183 B2). Due to the loss of up to 40 g of albumin per treatment session, high cut-off membranes so far have been used for acute applications only, although some physicians have contemplated benefits of using them in chronic applications, possibly in conjunction with albumin substitution and/or in addition to or in alternate order with standard high-flux dialyzers. The expression “high cut-off membrane” or “high cut-off membranes” as used herein refers to membranes having a MWRO of between 15 and 20 kDa and a MWCO of between 170-320 kDa. The membranes can also be characterized by a pore radius, on the selective layer surface of the membrane, of between 8-12 nm. For the avoidance of doubt, the determination of MWRO and MWCO for a given membrane and as used herein is according to the methods of Boschetti-de-Fierro et al. (2013); see “Materials and Methods” section of the reference and Example 3 of this description. Accordingly, the expressions “as determined by dextran sieving” or “based on dextran sieving” also refer to the dextran sieving method as described in Boschetti-de-Fierro et al. (2013) and as further described herein (Example 3). Processes for producing high cut-off membranes have been described, for example, in the aforementioned references. As disclosed already in WO 2004/056460 A1, a key element for their generation is a careful control of the temperature of the spinning process, i.e. the temperature of the spinneret, the spinning shaft temperature and temperature of the coagulation bath, relative to the spinning conditions for producing a high-flux membrane with about the same composition of polymers. In addition, for the production of the latest high cut-off membranes such as the Theralite® membrane, the ratio of water and solvent (H₂O/solvent) in the polymer solution is also slightly changed to lower values while the polymer content in said solution can otherwise be similar to or the same as used for producing high-flux membranes such as, for example, the Revaclear® membrane.

The MWCO and MWRO values used for describing the prior art membranes and the membranes according to the invention have been measured before blood or plasma contact, because the sieving properties of synthetic membranes may change upon such contact. This fact can be attributed to the adhesion of proteins to the membrane surface, and is therefore related to the membrane material and the medium characteristics. When proteins adhere to the membrane surface, a protein layer is created on top of the membrane. This secondary layer acts also as a barrier for the transport of substances to the membrane, and the phenomenon is commonly referred to as fouling. The general classification and typical performance of blood purification membranes following the above reference is summarized in Table I.

TABLE I General classification and typical performance of hemodialysis membranes Sieveing Water Coefficient^(b) permeability^(a) β2- FLC Albumin Dialyzer ml/(m²hmm Micro- Clearance^(c) Loss type Hg) globulin Albumin Kappa Lambda (g)^(d) Low- 10-20 n.d. <0.01 n.d. n.d. 0 flux High- 200-400 0.7-0.8 <0.01 <10 <2 <0.5 flux Pro-  50-500 0.9-1.0 0.02- n.d. n.d. 2-6 tein 0.03 leak- ing High  862-1436 1.0 0.1-0.2 14-38 12-33 22-28 cut- off ^(a)with 0.9 wt.-% sodium chloride at 37 ± 1 ° C. and Q_(B) 100-500 ml/min ^(b)according to EN1283 with Q_(B) max and UF 20% ^(c)Serum Free Light Chains, Clearance in vitro, Q_(B) 250 ml/min and Q_(D) 500 ml/min, UF 0 ml/min, Bovine Plasma, 60 g/l, 37° C., Plasma Level: human κ 500 mg/1, human λ 250 mg/l. All clearances in ml/min, measured for membrane areas between 1.1 and 2.1 m² ^(d)measured in conventional hemodialysis, after a 4-h session, with Q_(B) 250 ml/min and Q_(D) 500 ml/min, for membrane areas between 1.1 and 2.1 m².

As already mentioned, sieving curves give relevant information in two dimensions: the shape of the curve describes the pore size distribution, while its position on the molecular weight axis indicates the size of the pores. The molecular weight cut-off (MWCO) limits the analysis of the sieving curve to only one dimension, namely to the size of the pores where the sieving coefficient is 0.1. To enhance membrane characterization, the molecular weight retention onset (MWRO) is used herein for characterizing the membranes according to the invention. By using both MWCO and MWRO it becomes evident how the membranes of the invention distinguish themselves from prior art membranes for typical representatives of which MWCO and MWRO have been determined under the same conditions.

The MWRO is defined as the molecular weight at which the sieving coefficient is 0.9 (see FIG. 4 of Boschetti-de-Fierro et al (2013)). It is otherwise analogous to the MWCO but describes when the sieving coefficient starts to fall. Defining two points on the sieving curves allows a better, more concise characterization of the sigmoid curve, giving an indication of the pore sizes and also of the pore size distribution and thus of the most relevant physical parameters which determine a membrane. The expression “molecular weight retention onset”, “MWRO” or “nominal molecular weight retention onset” as interchangeably used herein therefore refers to the molecular mass of a solute where the membranes have a rejection of 10%, or, in other words, allow passage of 90% of the solute, corresponding to a sieving coefficient of 0.9. The dextran data from molecular weight fractions is also directly related to the size of the molecules and is an indirect measure of the pore sizes in the membranes. Thus, the MWRO is also directly related to a physical property of the membrane. One can interpret this value as some reference of where the pore size distribution starts, while the MWCO indicates where it ends.

The use of dextran sieving curves together with the respective MWCO and MWRO values based thereon allows differentiating the existing dialyzer types low-flux, high-flux, protein leaking, or high cut-off (see FIG. 5 of Boschetti-Fierro et al. (2013)) and the new and improved membranes which are described herein. Compared, for example, to the high-flux dialyzers, which are the standard for current dialysis treatment, the low-flux dialyzers are depicted in a group with low MWRO and MWCO (FIG. 2 ). The other two known families—protein leaking and high cut-off dialyzers—have different characteristics. While the protein leaking dialyzers are mainly characterized by a high MWCO and a low MWRO, the high cut-off family can be strongly differentiated due to the high in vitro values for both MWRO and MWCO (Table II).

TABLE II General classification of current hemodialysis membranes based on dextran sieving Structural Characteristics Dialyzer MWRO MWCO Pore radius type [kDa] [kDa] [nm] Low-flux 2-4 10-20 2-3 High-flux  5-10 25-65 3.5-5.5 Protein 2-4 60-70 5-6 leaking High cut-off 15-20 170-320  8-12

It is obvious from FIG. 5 of Boschetti et al. (2013) that there exists a gap between the currently known high cut-off and high-flux membranes, which so far could not be addressed by currently available membranes. Membranes which would be located in this gap would, however, be highly desirable, as they would form the nexus between an increasingly important removal of larger uremic solutes as realized in present high cut-off membranes, and a sufficient retention of albumin and other essential proteins which currently puts a limit to an even broader usability of the beneficial characteristics of high cut-off membranes, for example in chronic applications. However, to date, no such membranes have been described or prepared, even though continuous attempts have been made to produce such membranes (see, for example, EP 2 253 367 A1). So far, no available membrane was able to fulfil the above described expectations as regards MWRO and MWCO. Membranes which are coming close to the said gap (EP 2 253 367 A1) could be prepared only by means of processes which are not feasible for industrial production.

SUMMARY

It was the object of the present invention to develop a class of membranes with enhanced sieving properties, allowing removal of middle and large uremic solutes which cannot be addressed by the current membranes under acceptable albumin losses for chronic patients, and which can be prepared by industrially feasible production processes, specifically without treating the membranes with a salt solution before drying as described in EP 2 243 367 A1. In the present invention, semipermeable membranes are disclosed which are characterized by a molecular retention onset (MWRO) of between 9.0 kDa and 14.0 kDa and a molecular weight cut-off (MWCO) of between 55 kDa and 130 kDa as determined by dextran sieving curves before the membrane has had contact with blood or a blood product, and wherein during production of the semipermeable membrane said membrane is not treated with a salt solution before drying.

As a result, the new, industrially producible membranes significantly extend the removable range of uremic solutes while sufficiently retaining albumin for safe use in chronic applications with patients suffering from renal failure (FIG. 1 ). The membranes in the context of the present invention are polysulfone-based, polyethersulfone-based or poly(aryl)ethersulfone-based synthetic membranes, comprising, in addition, a hydrophilic component such as, for example, PVP and optionally low amounts of further polymers, such as, for example, polyamide or polyurethane. The present invention is also directed to a method of preparing such membranes, wherein the spinning temperature is increased relative to spinning temperatures chosen for obtaining polysulfone-based, polyethersulfone-based or poly(aryl)ethersulfone-based synthetic high-flux membranes with a given polymer composition, and by increasing, at the same time, the ratio of H₂O and solvent in the center solution relative to the ratios which would otherwise be used for obtaining polysulfone-based, polyethersulfone-based or poly(aryl)ethersulfone-based synthetic high cut-off membranes. In contrast to similar membranes such as disclosed in EP 2 243 367 A1, the present membranes can be prepared without treating the membranes of the invention with a salt solution before the drying step, which in addition to having made accessible a process which can be used on industrial scale leads to membranes which show even more pronounced advantages with regard to MWCO and MWRO. The present invention is also directed to methods of using the membrane in blood purification applications, in particular in hemodialysis methods for treating advanced and permanent kidney failure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general, schematic representation of small, middle and large molecular solutes which are removed by various blood purification membranes and operation modes in comparison. HD represents hemodialysis. HDF represents hemodiafiltration. The largest molecules will be removed by high cut-off membranes (hemodialysis mode). High-flux membranes, in hemodialysis mode, are able to remove small molecules and certain middle molecules in HD, whereas the same membranes will remove larger middle molecules in hemodiafiltration mode. The membranes according to the invention are able to remove also large molecules such as IL-6 and λ-FLC in hemodialysis mode. Essential proteins like, for example, albumin are essentially retained.

FIG. 2 shows the results of dextran sieving measurements wherein the MWRO (molecular weight retention onset) is plotted against the MWCO (molecular weight cut-off). Each measuring point represents three dextran sieving measurements of a given membrane. Dextran sieving measurements were performed according to Example 3. The respective MWCO and MWRO values were measured and the average value for a given membrane was entered into the graph shown. The membranes marked with a triangle (▴) and contained in two squares of varying sizes are membranes according to the invention and have been prepared in general accordance with what is disclosed in Example 1. The data points outside the square(s) are prior art membranes which are either low-flux membranes (●; a-c), high-flux membranes (◯; 1-13), high cut-off membranes (Δ; α, β, γ, ϕ)) or so-called protein-leaking membranes (▾). It is evident from the graph that the membranes according to the invention (▴; A-G) form a group of membranes which in the representation of MWRO against MWCO is located between the high-flux and high cut-off membranes of the prior art. The respective membranes, the processes for preparing them and/or their identity are provided for in further detail in Example 1.

FIG. 3 is a schematic representation of the experimental setup for the filtration experiments according to Example 3, showing: (1) pool with dextran solution, (2) feed pump, (3) manometer, feed side Pin, (4) manometer, retentate side Pout, (5) manometer, filtrate side PUF, (6) filtrate pump (with less than 10 ml/min), (7) heating/stirring plate.

FIG. 4 exemplarily shows the dextran sieving curves for a selection of membranes taken from different classes. The sieving curve of Membrane A (−) (Example 1.1) according to the invention in comparison with high cut-off type membranes of the prior art (Membrane β and Membrane ϕ, Examples 1.8 and 1.11, respectively) shows about the same ability to more efficiently remove molecules with a higher molecular weight compared to, for example, high-flux Membrane 6 (Example 1.17). At the same time, Membrane A shows a steeper slope than Membrane β and Membrane ϕ, demonstrating a more efficient retention of albumin compared to high cut-off membranes.

FIG. 5A to F exemplarily show scanning electron micrographs of Membrane A according to the invention. Magnifications used are indicated in each Figure. FIG. 5A shows a profile of the hollow fiber membrane, whereas FIG. 5B a close-up cross-section through the membrane, where the overall structure of the membrane is visible. FIGS. 5C and 5D represent further magnifications of the membrane wall, wherein the inner selective layer is visible. FIG. 5E shows the inner selective layer of the membrane, FIG. 5F shows the outer surface of the hollow fiber membrane.

FIG. 6A to F exemplarily show scanning electron micrographs of Membrane F according to the invention. Magnifications used are indicated in each Figure. FIG. 6A shows a profile of the hollow fiber membrane, whereas FIG. 6B a close-up cross-section through the membrane, where the overall structure of the membrane is visible. FIGS. 6C and 6D represent further magnifications of the membrane wall, wherein the inner selective layer is visible. FIG. 6E shows the inner selective layer of the membrane, FIG. 6F shows the outer surface of the hollow fiber membrane.

DETAILED DESCRIPTION

Middle molecules, consisting mostly of peptides and small proteins with molecular weights in the range of 500-60,000 Da, accumulate in renal failure and contribute to the uremic toxic state. These solutes are not well cleared by low-flux dialysis. High-flux dialysis will clear middle molecules, partly by internal filtration. Many observational studies over the last years have indeed supported the hypothesis that higher molecular weight toxins (FIG. 1 ) are responsible for a number of dialysis co-morbidities, including, for example, chronic inflammation and related cardiovascular diseases, immune dysfunctions, anaemia etc., influencing also the mortality risk of chronic hemodialysis patients. It is possible to enhance the convective component of high-flux dialysis by haemodiafiltration (HDF). However, in case of postdilution HDF, increasing blood flow above the common routine values may create problems of vascular access adequacy in many routine patients and is therefore not accessible to all patients in need. Predilution HDF allows for higher infusion and ultrafiltration rates. However, this advantage in terms of convective clearances is thwarted by dilution of the solute concentration available for diffusion and convection, resulting in the reduction of cumulative transfer. Therefore, there is an increasing interest in developing new membranes which in hemodialysis mode allows an enhanced transport of middle and even large molecules, comparable or superior to high-flux membranes when used in HDF mode, while at the same time efficiently retaining albumin and larger essential proteins such as coagulation factors, growth factors and hormones. In short, such desired membranes should still better reproduce the physiological glomerular ultrafiltration compared to membranes already available today.

Semipermeable membranes are now provided which are suitable for blood purification in hemodialysis mode, and which have an increased ability to remove larger molecules which is comparable or superior to hemodiafiltration, while at the same time albumin is efficiently retained. The membranes are characterized by a molecular retention onset (MWRO) of between 9.0 kDa and 14.0 kDa and a molecular weight cut-off (MWCO) of between 55 kDa and 130 kDa as determined by dextran sieving (FIG. 2 ), with the proviso that membranes are excluded which have been prepared by treating the membranes with a salt solution before drying as described in EP 2 243 367 A1. Thus, according to one aspect of the present invention, the membranes are characterized by a MWRO of between 9000 and 14000 Daltons as determined by dextran sieving measurements, which indicates that the membranes according to the invention have the ability to let pass 90% of molecules having a molecular weight of from 9.0 to 14.0 kDa. Notably, said MWRO is achieved in hemodialysis (HD) mode. The molecules of said molecular weight range belong to the group of molecules generally referred to as middle molecules which otherwise can only efficiently be removed by certain high cut-off membranes at the cost of some albumin loss or by certain high-flux membranes which are used in HDF mode. According to another aspect of the invention, the membranes are further characterized by a MWCO of between 55 kDa and 130 kDa Daltons as determined by dextran sieving, which indicates that the membranes are able to effectively retain larger blood components such as albumin (67 kDa) and molecules larger than said albumin. In contrast, the average MWRO range of high-flux membranes lies in the range of from about 4 kDa to 10 kDa as determined by dextran sieving, combined with a MWCO of from about 19 kDa to about 65 kDa as determined by dextran sieving. High cut-off membranes are characterized by a significantly higher MWCO, as determined by dextran sieving, of from about 150-320 kDa, and a MWRO, as determined by dextran sieving of between 15-20 kDa.

According to another aspect of the present invention, the membranes of the invention have a MWRO, as determined by dextran sieving, in the range of from 9.0 kDa to 12.5 kDa and a MWCO, as determined by dextran sieving, in the range of from 55 kDa to 110 kDa. According to another aspect of the present invention, the membranes of the invention have a MWRO, as determined by dextran sieving, in the range of from 9.0 kDa to 12.5 kDa and a MWCO, as determined by dextran sieving, in the range of from 68 kDa to 110 kDa. According to yet another aspect of the present invention, the membranes have a MWRO, as determined by dextran sieving, in the range of from 10 kDa to 12.5 kDa and a MWCO, as determined by dextran sieving, in the range of from 68 kDa to 90 kDa. According to yet another aspect of the present invention, membranes have a MWRO, as determined by dextran sieving, of more than 10.0 kDa but less than 12.5 kDa and a MWCO, as determined by dextran sieving, of more than 65.0 kDa and less than 90.0 kDa.

As mentioned before, the membranes of the invention are able to control albumin loss and loss of other essential higher molecular weight blood components. In general, the membranes of the invention limit the protein loss in vitro (bovine plasma with a total protein concentration of 60±5 g/l, Q_(B)=300 ml/min, TMP=300 mmHg) after 25 minutes to a maximum of 1.0 to 2.0 g/l in a dialysis filter having an effective membrane area of 1.8 m². According to one embodiment of the invention the membranes of the invention limit the protein loss in vitro (bovine plasma with a total protein concentration of 60±5 g/l, Q_(B)=300 ml/min, TMP=300 mmHg) after 25 minutes to a maximum of 1.2 to 1.4 g/l in a dialysis filter having an effective membrane area of 1.8 m². According to another aspect of the present invention, the membrane of the invention will limit albumin loss per treatment (240 min±20%) with a hemodialysis filter comprising said membrane, a blood flow of between 200-600 ml/min, a dialysate flow of between 300-1000 ml/min and an ultrafiltration rate of between 0 and 30 ml/min to a maximum of 10 g (Example 5). According to one aspect of the present invention, the albumin loss under the same conditions is limited to 7 g. According to yet another aspect of the present invention, the albumin loss under the same conditions is limited to 4 g. According to one embodiment of the invention, the ultrafiltration rate used with a hemodialyzer comprising the membrane of the invention is between 0 and 20 ml/min. According to another embodiment of the invention, the ultrafiltration rate used with a hemodialyzer comprising the membrane of the invention is between 0 and 15 ml/min. According to yet another embodiment of the invention, the ultrafiltration rate is 0 ml/min. The blood flow range used with a hemodialyzer comprising the membrane of the invention according to another embodiment of the invention will be in the range of between 350-450 ml/min, and the dialysate flow will be in the range of from between 500 and 800 ml/min.

Membrane passage of a solute such as a protein which needs to be removed from blood or needs to be retained, as the case may be, is described by means of the sieving coefficient S. The sieving coefficient S is calculated according to S=(2C_(F))/(C_(Bin)+C_(Bout)), where C_(F) is the concentration of the solute in the filtrate and C_(Bin) is the concentration of a solute at the blood inlet side of the device under test, and C_(Bout) is the concentration of a solute at the blood outlet side of the device under test. A sieving coefficient of S=1 indicates unrestricted transport while there is no transport at all at S=0. For a given membrane each solute has its specific sieving coefficient. For example, the membranes according to the invention have an average sieving coefficient for albumin, measured in bovine plasma according to DIN EN ISO8637:2014 at Q_(B)=400 ml/min and UF=25 ml/min (see also Example 4) of between 0.01 and 0.2. According to another aspect of the invention, the membranes according to the invention have an average sieving coefficient for albumin, measured in bovine plasma according to DIN EN ISO8637:2014 at Q_(B)=400 ml/min and UF=25 ml/min of between 0.02 and 0.1. According to yet another aspect of the invention, the membranes according to the invention have an average sieving coefficient for albumin, measured in bovine plasma according to DIN EN ISO8637:2014 at Q_(B)=400 ml/min and UF=25 ml/min of between 0.02 and 0.08. According to another aspect of the invention, the membranes according to the invention have an average sieving coefficient for albumin, measured in bovine plasma according to EN1283 at Q_(B)=600 ml/min and UF=120 ml/min of between 0.01 and 0.1. According to yet another aspect of the invention, the membranes according to the invention have an average sieving coefficient for albumin, measured in bovine plasma according to EN1283 at Q_(B)=600 ml/min and UF=120 ml/min of between 0.01 and 0.06.

The semipermeable hemodialysis membrane according to the invention comprises at least one hydrophilic polymer and at least one hydrophobic polymer. In one embodiment, said at least one hydrophilic polymer and at least one hydrophobic polymer are present as coexisting domains on the surface of the dialysis membrane. According to one embodiment of the invention, the polymer solution may contain an additional hydrophobic polymer, such as, for example, polyamide, which is added to the polymer composition in low amounts.

The hydrophobic polymer may be chosen from the group consisting of poly(aryl)ethersulfone (PAES), polysulfone (PSU) and polyethersulfone (PES) or combinations thereof. In a specific embodiment of the invention, the hydrophobic polymer is chosen from the group consisting of poly(aryl)ethersulfone (PAES) and polysulfone (PSU). The hydrophilic polymer will be chosen from the group consisting of polyvinylpyrrolidone (PVP), polyethyleneglycol (PEG), polyvinylalcohol (PVA), and a copolymer of polypropyleneoxide and polyethyleneoxide (PPO-PEO). In another embodiment of the invention, the hydrophilic polymer may be chosen from the group consisting of polyvinylpyrrolidone (PVP), polyethyleneglycol (PEG) and polyvinylalcohol (PVA). In one specific embodiment of the invention, the hydrophilic polymer is polyvinylpyrrolidone (PVP).

The membranes according to the invention can be produced as flat sheet membranes or as hollow fiber membranes. Flat sheet membranes can be produced according to methods known in the art. Preferably, the dialysis membrane according to the invention is a hollow fiber membrane having an asymmetric foam- or sponge-like and/or a finger-like structure with a separation layer present in the innermost layer of the hollow fiber. According to one embodiment of the invention, the membrane of the invention has an asymmetric sponge-like structure (FIG. 6 ). In another embodiment of the invention, the membrane of the invention has an asymmetric finger-like structure with at least three layers, wherein the separation layer has a thickness of less than 0.5 μm. In one embodiment, the separation layer contains pore channels having an average effective pore size (radius) before blood contact of between about 5.0 and 7.0 nm as determined from the MWCO based on dextran sieving coefficients according to Boschetti-de-Fierro et al. (2013) and Granath et al. (1967). The average effective pore size (radius) of this membrane type before blood contact is generally above 5.0 nm and below 7.0 nm, and specifically above 5.0 nm and below 6.7 nm. The next layer in the hollow fiber membrane is the second layer, having the form of a sponge structure and serving as a support for said first layer. In a preferred embodiment, the second layer has a thickness of about 1 to 15 μm. The third layer has the form of a finger structure. Like a framework, it provides mechanical stability on the one hand; on the other hand a very low resistance to the transport of molecules through the membrane, due to the high volume of voids. The third layer, in one embodiment of the invention, has a thickness of 20 to 30 μm. In another embodiment of the invention, the membranes also include a fourth layer, which is the outer surface of the hollow fiber membrane. This fourth layer has a thickness of about 1 to 10 μm. As can easily be understood, a combination of the above ranges will always add up to a wall thickness within the aforementioned ranges for wall thicknesses of the hollow fiber membranes in accordance with the present invention.

The manufacturing of a membrane according to the invention follows a phase inversion process, wherein a polymer or a mixture of polymers is dissolved in a solvent to form a polymer solution. The solution is degassed and filtered before spinning. The temperature of the polymer solution is adjusted during passage of the spinning nozzle (or slit nozzle) whose temperature can be regulated and is closely monitored. The polymer solution is extruded through said spinning nozzle (for hollow fibers) or a slit nozzle (for a flat film) and after passage through the so-called spinning shaft enters into said precipitation bath containing a nonsolvent for the polymer and optionally also a solvent in a concentration of up to 20 wt.-%. To prepare a hollow fiber membrane, the polymer solution preferably is extruded through an outer ring slit of a nozzle having two concentric openings. Simultaneously, a center fluid is extruded through an inner opening of the spinning nozzle. At the outlet of the spinning nozzle, the center fluid comes into contact with the polymer solution and at this time the precipitation is initialized. The precipitation process is an exchange of the solvent from the polymer solution with the non-solvent of the center fluid. By means of this exchange the polymer solution inverses its phase from the fluid into a solid phase. In the solid phase the pore structure and the pore size distribution is generated by the kinetics of the solvent/non-solvent exchange. The process works at a certain temperature which influences the viscosity of the polymer solution. For preparing membranes according to the invention, the temperature of the spinning nozzle and, consequently, of the polymer solution and the center fluid as well as the temperature of the spinning shaft should be carefully controlled. In principle, membranes of the invention can be prepared at a comparatively broad temperature range. Temperature may thus be in the range of between 30 and 70° C. However, for producing a membrane of the invention, the ultimate temperature should be chosen by taking account of the polymer composition and the temperature which would otherwise be used for producing a standard high-flux membrane with about the same polymer composition and which can be used as a starting point for the production of a membrane according to the invention. In general, there are two parameters which can be effectively influenced to arrive at membranes of the present invention. First, the temperature at the spinning nozzle should be slightly raised within a range of from 0.5° C. to 4° C. relative to the temperatures used for producing the common high-flux type membranes having about the same polymer composition, resulting in a corresponding increase of the temperature of the polymer solution. Second, the water content in the center solution should be slightly reduced in a range of from 0.5 wt.-% to 4 wt.-%, preferably from 0.5 to 3 wt.-%. It should be obvious that the polymer composition for preparing a membrane according to the invention does not have to be completely identical to a typical polymer composition for preparing a high-flux membrane, such as, for example, Membrane 6. Accordingly, expressions such as “about the same polymer composition” as used in the present context refers to polymer compositions having the same basic composition, for example, a combination of PS, PES or PAES on the one hand and PVP on the other hand, in concentrations typically used for the production of high-flux type membranes and/or membranes according to the present invention.

As mentioned before, the temperature influences the viscosity of the spinning solution, thereby determining the kinetics of the pore-forming process through the exchange of solvent with non-solvent. The viscosity of a spinning solution for preparing membranes according to the invention generally should be in the range of from 3000 to 7400 mPas at 22° C. According to one embodiment of the invention, the viscosity is in the range of from 4900 to 7400 mPas (22° C.). According to yet another embodiment of the invention the viscosity will be in the range of from 4400 to 6900 mPas (22° C.). For arriving at foam- or sponge-like structures the viscosity can, for example, be increased to values of up to 15000 mPas, even though such structures can also be obtained with lower values in the above-stated ranges.

Another aspect of preparing a membrane according to the invention concerns the temperature of the center fluid. The center fluid generally comprises 45 to 60 wt.-% of a precipitation medium, chosen from water, glycerol and other alcohols, and 40 to 55 wt.-% of solvent. In other words, the center fluid does not comprise any hydrophilic polymer.

The temperature of the center fluid is in principle the same as the temperature chosen for the spinning nozzle as the temperature of the center fluid will be determined when it passes through said nozzle. According to one embodiment of the invention, the center fluid is composed of water and NMP, wherein the water is present in a concentration of from 50 to 58 wt.-%.

According to a further embodiment of the invention, the polymer solution coming out through the outer slit openings is, on the outside of the precipitating fiber, exposed to a humid steam/air mixture. Preferably, the humid steam/air mixture in the spinning shaft has a temperature of between 50° C. to 60° C. According to one embodiment of the invention, the temperature in the spinning shaft is in the range of from 53° C. to 58° C. The distance between the slit openings and the precipitation bath may be varied, but generally should lie in a range of from 500 mm to 1200 mm, in most cases between 900 mm and 1200 mm. According to one embodiment of the invention the relative humidity is >99%.

According to another aspect of the present invention, following passage through the spinning shaft the hollow fibers enter a precipitation bath which generally consists of water having a temperature of from 12° C. to 30° C. For preparing the membranes according to the invention, the temperature of the precipitation bath may be slightly elevated by 1 to 10° C. in comparison to the temperature which would otherwise be chosen for preparing a high-flux or high cut-off membrane. According to one embodiment of the invention an increase by 2° C. to 10° C. and more specifically an increase of up to 6° C. may be recommendable to arrive at membranes of the present invention.

According to one specific embodiment of the invention, the temperature of the precipitation bath is between 23° C. and 28° C. The membrane according to the present invention will then be washed in consecutive water baths to remove waste components, and can then be directly submitted, for example, to online drying at temperatures of between 150° C. to 280° C.

In order to illustrate what has been said before, a membrane according to the invention can be produced, for example, as follows. For a composition based on poly(aryl)ethersulfone, polyethersulfone or polysulfone and PVP, the temperature of the spinning nozzle, for example, can be chosen to be in a range of from 56° C. to 59° C., and the temperature of the spinning shaft is then in the range of from 53° C. to 56° C. in order to reliably arrive at a membrane according to the invention. Preferably, the temperature of the spinning nozzle is in the range of from 57° C. to 59° C., more preferably in a range of from 57° C. to 58° C., and the temperature in the spinning shaft is then in the range of from 54° C. to 56° C. In each case the viscosity of the spinning solution after preparation should be in the range of from 3000 to 7400 mPas at 22° C. Such composition, may, for example, comprise between 12 and 15 wt.-% of poly(aryl)ethersulfone, polyethersulfone or polysulfone, between 5 and 10 wt.-% of PVP, between 72 and 81 wt.-% of a solvent, such as NMP, and between 2 and 3 wt.-% of water. A typical composition thus would comprise 14 wt.-% of poly(aryl)ethersulfone, polyethersulfone or polysulfone, 7 wt.-% of PVP, 77 wt.-% of a solvent and 2 wt.-% water. At the same time, the center solution should comprise, for example, 54.0 to 55 wt.-% water and 46.0 to 45.0 wt.-% solvent, e.g. NMP, respectively. For example, the center solution may contain 54.5% water and 45.5 solvent, such as NMP.

The spinning velocity often may influence the properties of the resulting membranes. In the present case, the velocity may be chosen to be in a relatively broad range from about 10 to 60 m/min without departing from the invention, even though higher spinning velocities which still provide for a stable production process will be desirable for economic reasons. According to one embodiment of the invention, the spinning velocity for arriving at membranes according to the invention will therefore be in the range of from 30 to 50 m/min. According to another embodiment of the invention, the spinning velocity for arriving at membranes as used for accomplishing hemodialyzers according to the invention will be in the range of from 40 to 55 m/min.

According to one embodiment of the invention, the polymer solution used for preparing the membrane preferably comprises 10 to 20 wt.-% of the hydrophobic polymer, 2 to 11 wt.-% of the hydrophilic polymer, as well as water and a solvent, such as, for example, NMP. Optionally, low amounts of a second hydrophobic polymer can be added to the polymer solution. The spinning solution for preparing a membrane according to the present invention preferably comprises between 12 and 15 weight-% of polyethersulfone or polysulfone as hydrophobic polymer and 5 to 10 weight-% of PVP, wherein said PVP may consist of a low and a high molecular PVP component. The total PVP contained in the spinning solution thus may consist of between 22 and 34 weight-% and preferably of between 25 and 30 weight-% of a high molecular weight component and of between 66 and 78 weight-%, preferably of between 70 and 75 weight-% of a low molecular weight component. Examples for high and low molecular weight PVP are, for example, PVP K85/K90 and PVP K30, respectively. The solvent may be chosen from the group comprising N-methylpyrrolidone (NMP), dimethyl acetamide (DMAC), dimethyl sulfoxide (DMSO) dimethyl formamide (DMF), butyrolactone and mixtures of said solvents. According to one embodiment of the invention, the solvent is NMP.

As mentioned before, the type, amount and ratio of hydrophilic and hydrophobic polymers used for producing membranes according to the invention may be similar to or the same as those which would otherwise be used for the production of high-flux membranes which are known in the art. It is, however, important for arriving at membranes according to the invention to adjust the ratio of water and solvent (H₂O/solvent) in the polymer solution compared to standard high-flux recipes to slightly lower values, i.e. to slightly decrease the total concentration of water in the polymer solution by about 0.5 wt.-% to 4 wt.-% and to adjust the amount of solvent accordingly by slightly increasing the total concentration of the respective solvent. In other words, in a given polymer composition, the amount of water will be slightly reduced and the amount of solvent will at the same time and rate be slightly increased compared to polymer compositions used for standard high-flux membranes.

As an alternative way to arrive at membranes according to the invention it is also possible to choose, as a starting point, known recipes and processes for preparing high cut-off membranes. In this case, the polymer composition, including water and solvent, will generally remain about the same as a composition typically used for preparing high cut-off membranes, such as shown for Membranes α or β. However, the ratio of H₂O and solvent in the center solution should be increased as compared to the typical center solution used for preparing a high cut-off membrane, such as, for example, for Membranes α and β, i.e. the water content is slightly increased by about 0.5 wt.-% to 4.0 wt.-%.

The slight increase in the water content in the center solution should be accompanied by an adaption of the spinning nozzle and spinning shaft temperature. An increase in water content will generally be accompanied by appropriately adapting the temperature of the spinneret and the spinning shaft by up to 4° C., preferably by about between 0.5° C. to 3° C. relative to the respective temperatures used for producing a high cut-off type membrane. Depending on the aspired characteristics of the membranes according to the invention in terms of MWRO and MWCO values, the change in the water content of the center solution can be accompanied, for example, by a temperature increase of spinneret and spinning shaft of up to 4° C., preferably by 0.5° C. to 3° C., resulting in rather open-pored membrane species which would be located in the upper right corner of the square shown in FIG. 2 . It may also be accompanied by a slight decrease of the spinneret's and spinning shaft's temperature by about 0.5° C. to 3° C., preferably by 0.5° C. to 2° C., respectively, resulting in a less open-pored, more high-flux like membrane species according to the invention, which would then be located in the lower left corner of the square shown in FIG. 2 .

Accordingly, it is one aspect of the present invention, that the membranes according to the invention can be obtained by dissolving at least one hydrophobic polymer component and at least one hydrophilic polymer in at least one solvent to form a polymer solution having a viscosity of from 3000 to 7400 mPas at a temperature of 22° C., extruding said polymer solution through an outer ring slit of a spinning nozzle with two concentric openings and extruding a center fluid comprising at least one solvent and water through the inner opening of the nozzle, passing the polymer solution through a spinning shaft into a precipitation bath, wherein the distance between the slit openings and the precipitation bath is between 500 mm to 1200 mm, preferably between 900 mm and 1200 mm, and wherein the relative humidity of the steam/air mixture in the spinning shaft is between 60% and 100%, washing the membrane obtained, drying said membrane and, optionally, sterilizing said membrane by steam treatment, wherein the content of water in the center solution is increased by between 0.5 wt.-% and 4 wt.-% relative to the water content which is used for preparing a high-cut off membrane having the same polymer composition, and wherein the temperature of the spinning nozzle and the spinning shaft is either decreased by up to 3° C., preferably by 0.5° C. to 2° C., relative to the temperature which would be used for preparing a high-cut off membrane having the same polymer composition, or is increased by 0.5° C. to 4° C., preferably 0.5° C. to 3° C., relative to the temperature which would be used for preparing a high-cut off membrane having the same polymer composition, or essentially remains the same.

The membrane after washing and without being immersed in any salt bath can directly be submitted to a drying step, such as online drying, and is then preferably steam sterilized at temperatures above 121° C. for at least 21 minutes. It is, however, also possible to use other methods known in the art for sterilizing the membrane and/or the filter device comprising same.

A membrane according to the invention which is based on, for example, poly(aryl)ethersulfone and PVP, after preparation comprises from between 2.0 wt.-% to 4.0 wt.-% PVP and poly(aryl)ethersulfone adding up to 100%, respectively.

Hollow fiber membranes according to the invention can be produced with different inner and outer diameters and the wall thickness of such hollow fiber membranes may vary over a certain range. High cut-off membranes known in the art, such as Theralite® and HCO1100®, have a comparatively large inner diameter of the fiber of 215 μm and a wall thickness of 50 μm. Known high-flux membranes such as used, for example, in the Revaclear®400 filter have inner diameters of 190 μm and a wall thickness of 35 μm, or, in the case of the FX CorDiax hemodiafilters, an inner diameter of 210 μm. Membranes according to the invention are preferably prepared with a wall thickness of below 55 μm, generally with a wall thickness of from 30 to 49 μm. The membranes can, however, be produced with a wall thickness of below 40 μm, generally in the range of about 30 to 40 μm, such as, for example, with a wall thickness of 35 μm. The inner diameter of the hollow fiber membranes of the present invention may be in the range of from 170 μm to 200 μm, but may generally be reduced to below 200 μm or even below 190 μm, for example to about 175 μm to 185 μm for full efficiency in the context of the present invention.

The membranes used in hemodialyzers according to the invention can be further characterized by an average sieving coefficient for β2-M, measured in bovine plasma (total protein 60±5 g/l total protein) according to EN1283 (Q_(B)max, UF=20%) with blood flow rates of between 400 ml/min and 600 ml/min of between 0.7 and 1 (Example 4). According to another embodiment of the invention the sieving coefficients for β2-M under the same conditions are between 0.8 and 1. According to yet another embodiment of the invention the sieving coefficients for β2-M under the same conditions are between 0.9 and 1. According to another embodiment of the invention the sieving coefficients for β2-M measured according to DIN EN ISO8637:2014 at Q_(B)=400 ml/min and UF=25 ml/min are between 0.8 and 1. According to yet another embodiment of the invention the sieving coefficients for β2-M under the same conditions are between 0.9 and 1.

The membranes can also be characterized by an average sieving coefficient for myoglobin, measured in bovine plasma according to EN1283 (Q_(B)max, UF=20%) with blood flow rates of between 400 ml/min and 600 ml/min of between 0.7 and 1 (Example 4). According to another embodiment of the invention the sieving coefficients for myoglobin under the same conditions are between 0.8 and 1, more specifically between 0.9 and 1. According to another embodiment of the invention the sieving coefficients for myoglobin, measured according to DIN EN ISO8637:2014 at Q_(B)=400 ml/min and UF=25 ml/min are between 0.8 and 1. According to yet another embodiment of the invention the sieving coefficients for myoglobin under the same conditions are between 0.9 and 1.

Due to their specific combination of MWRO and MWCO, the membranes according to the invention are especially beneficial for the treatment of chronic, but also of acute renal failure by hemodialysis. Their new features allow the highly efficient removal of uremic molecules having a medium to large molecular weight (FIG. 1 ) by hemodialysis, whereas state of the art membranes achieve a similar performance only in HDF treatment modes.

The blood flow rates which can be used with devices comprising the membranes according to the invention are in the range of from 200 ml/min to 600 ml/min. Dialysate flow rates for use with the membranes according to the invention are in the range of from 300 ml/min to 1000 ml/min. Usually, blood flow rates of from 300 ml/min to 500 ml/min, dialysis flow rates of from 500 ml/min to 800 ml/min and UF rates of from 0 to 15 ml/min will be used. For example, a standard flow rate used is Q_(B)=300 ml/min, Q_(D)=500 ml/min and UF=0 ml/min.

It will be readily apparent to one skilled in the art that various substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention.

The present invention will now be illustrated by way of non-limiting examples in order to further facilitate the understanding of the invention.

EXAMPLES Example 1

Preparation of Membranes

1.1 Membrane A

Two solutions were used for the formation of a membrane, the polymer solution consisting of hydrophobic and hydrophilic polymer components dissolved in N-methyl-pyrrolidone, and the center solution being a mixture of N-methyl-pyrrolidone (NMP) and water. The polymer solution contained poly(aryl)ethersulfone (PAES 14.0 wt-%) and polyvinylpyrrolidone (2 wt-% of PVP K85 and 5 wt-% of PVP K30, a total PVP concentration in the polymer solution of 7 wt-%). The solution further contained NMP (77.0 wt-%) and water (2.0 wt-%). The viscosity of the polymer solution, measured at a temperature of 22° C., was between 5500 and 5700 mPas. The spinneret was heated to a temperature of 59° C. The center solution contained water (54.5 wt-%) and NMP (45.5 wt-%). A defined and constant temperature regime was applied to support the process. The center solution was preheated to 59° C. and pumped towards the two-component hollow fiber spinneret. The polymer solution was leaving the spinneret through an annular slit with an outer diameter of 500 mm and an inner diameter of 350 mm/center solution slit 180 mm. The center fluid was leaving the spinneret in the center of the annular polymer solution tube in order to start the precipitation of the polymer solution from the inside and to determine the inner diameter of the hollow fiber. The two components (polymer solution and center fluid) were entering a space separated from the room atmosphere at the same time. This space is referred to as spinning shaft. A mixture of steam (˜100° C.) and air (22° C.) was injected into the spinning shaft. The temperature in the spinning shaft was adjusted by the ratio of steam and air to 56° C. The relative humidity of the steam is >99%. The length of the spinning shaft was 1050 mm. By the aid of gravity and a motor-driven roller, the hollow fiber was drawn from top to bottom, from spinneret through the spinning shaft into a water bath. The water bath had a temperature of 25° C. in vertical direction. The spinning velocity was 45 m/min. The hollow fiber was subsequently led through a cascade of water baths with temperatures increasing from about 25° C. to about 76° C. The wet hollow fiber membrane leaving the water-rinsing bath was dried in a consecutive online drying step. The hollow fiber was collected on a spinning wheel in the shape of a bundle. In some batches an additional texturizing step was added before the bundle was prepared. Alternatively, hand bundles according to Example 2 were formed for further experiments (see also FIG. 2 ). Scanning micrographs of the outer surface and of the hollow fiber according to Example 1.1 are shown in FIG. 5 . The membrane has a finger-like structure. The inner diameter of Membrane A was adjusted to be 180 μm and the wall thickness was chosen to be 35 μm.

1.2 Membrane B

Membrane B is based on the same polymer solution and center solution as Membrane A of Example 1.1 and was produced in analogy to what is described there. Differences were introduced only with regard to the temperature of the spinneret, which was adjusted to 58° C., the temperature of the spinning shaft, which was adjusted to 55° C. The temperature of the center solution was adjusted to 58° C. via the spinning nozzle.

1.3 Membrane C

Membrane C is based on the same polymer solution and center solution as Membrane A of Example 1.1 and was produced in analogy to what is described there. Differences were introduced only with regard to the temperature of the spinneret, which was adjusted to 57° C., and the temperature of the spinning shaft, which was adjusted to 54° C. The temperature of the center solution was adjusted to 57° C. via the spinning nozzle.

1.4 Membrane D

Membrane D is based on the same polymer solution and center solution as in Example 1.1 and was produced in analogy to what is described there. Differences were introduced only with regard to the polymer viscosity which in this case was 5071 mPas. The temperature of the center fluid was according to the spinning nozzle.

1.5 Membrane E

Membrane E is based on the same polymer solution and center solution as described in Example 1.1 and was produced in analogy to what is described there. In this case, the sieving data obtained slightly varied from data obtained with membranes prepared according to Example 1.1.

1.6 Membrane F

For obtaining sponge-like membrane structures, the polymer solution in contrast to Examples 1.1 to 1.5 contained a slightly different composition but was otherwise produced in analogy to what is described in Example 1.1. The solution contained poly(aryl)ethersulfone (PAES 14.0 wt-%) and polyvinylpyrrolidone (2 wt-% of PVP K85 and 5 wt-% of PVP K30). The solution further contained NMP (73.0 wt-%) and water (6.0 wt-%). The spinneret was heated to a temperature of 57° C. The center solution contained water (49.0 wt-%) and NMP (51.0 wt-%). The center solution was kept at 57° C. The temperature in the spinning shaft was adjusted to 55° C. The length of the spinning shaft was 1000 mm. The spinning velocity was 45 m/min. Scanning micrographs of the outer surface and of the hollow fiber according to Example 1.6 are shown in FIG. 6 . The inner diameter of Membrane F was again adjusted to be 180 μm and the wall thickness was again chosen to be 35 μm.

1.7 Membrane G

Membrane G was based on the same polymer solution as described in Example 1.6 (Membrane F) and was produced in analogy to what is described there. Differences were introduced with regard to the temperature of the spinneret, which was adjusted to 58° C., and the temperature of the spinning shaft, which was adjusted to 56° C. The temperature of the center solution was adjusted to 58° C. via the spinning nozzle. The inner diameter of Membrane G was again adjusted to be 180 μm and the wall thickness was again chosen to be 35 μm.

1.8 Comparative Example: High Cut-Off Membrane β

The polymer solution used for preparing a high cut-off Membrane β (see FIG. 2 ) according to the prior art was identical to the polymer solution used for the preparation of Membrane A (Example 1.1). However, the center solution used contained 53.0 wt.-% water and 47.0 wt.-% NMP. During the membrane formation process polymer and center solution were brought in contact with a spinneret and the membrane precipitated. The spinning velocity was 45 m/min. A defined and constant temperature regime was applied to support the process, wherein the spinneret was kept at a temperature of 58° C. The precipitated hollow fiber fell through a spinning shaft having a height of 1050 mm which was filled with steam (>99% relative humidity). The temperature within the shaft was stabilized to 54° C. Finally, the fiber entered a washing bath containing about 4 wt-% NMP in water, wherein the bath was kept a temperature of 20° C. The membrane was further washed in two additional water baths (75° C. and 65° C.) with counter current flow (250 l/h). Membrane drying was performed online, wherein remaining water was removed. The fibers had an inner diameter of 215 μm and a wall thickness of 50 μm.

1.9 Comparative Example: High Cut-Off Membrane α

The polymer solution and center solution as well as the process used for preparing the high cut-off Membrane α according to the prior art was identical to the polymer solution used for the preparation of Membrane β (Example 1.8). Differences existed with regard to the spinning velocity, which was lower than in Example 1.8 (29 m/min) and the online drying step, which in this case was omitted.

1.10 Comparative Example: High Cut-Off Membrane γ

The polymer solution and center solution as well as the process used for preparing the high cut-off Membrane γ according to the prior art was identical to the polymer solution used for the preparation of Membrane β (Example 1.8). Differences were introduced with regard to spinning velocity (34 m/min) and with regard to the temperature of the spinning shaft (56° C.)

1.11 Comparative Example: High Cut-Off Membrane ϕ

Membrane ϕ (FIG. 2 ) refers to hollow fiber membranes which were extracted from a Phylther® hemodialyzer (Phylther® HF 22 SD (2.2 m², Bellco, Italy)). The hollow fiber membranes are based on polyphenylene. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.12 Comparative Example: High-Flux Membrane 1

Membrane 1 (FIG. 2 ) refers to hollow fiber membranes which were extracted from a PES-21Dα^(eco) hemodialyzer (Nipro, Japan). The hollow fiber membranes are polyethersulfone based membranes (Polynephron®). The hollow fibers were used for preparing standardized mini-modules according to Example 3 for further tests.

1.13 Comparative Example: High-Flux Membrane 2

Membrane 2 (FIG. 2 ) refers to hollow fiber membranes which were extracted from an APS 21EA hemodialyzer (2.1 m², Asahi Kasei Medical Co., Ltd.). The hollow fiber membranes are polysulfone based membranes with a wall thickness of 45 μm and an inner diameter of 180 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.14 Comparative Example: High-Flux Membrane 3

Membrane 3 (FIG. 2 ) refers to hollow fiber membranes which were extracted from a Phylter® HF 17 G (1.7 m², Bellco, Italy)). The hollow fiber membranes are based on polyphenylene. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.15 Comparative Example: High-Flux Membrane 4

Membrane 4 (FIG. 2 ) refers to hollow fiber membranes which were extracted from a FX-S 220 filter (2.2 m², Fresenius Medical Care Japan KK) which is based on polysulfone and has a wall thickness of 35 μm and an inner diameter of 185 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.16 Comparative Example: High-Flux Membrane 5

Membrane 5 (FIG. 2 ) refers to hollow fiber membranes which were extracted from an Optiflux® F180NR filter (1.8 m², Fresenius Medical Care North America) which is based on polysulfone and has a wall thickness of 40 μm and an inner diameter of 200 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.17 Comparative Example: High-Flux Membrane 6

Membrane 6 (FIG. 2 ) refers to hollow fiber membranes which were prepared in accordance with Example 1 of EP 2 113 298 A1. The temperatures of the spinneret and the spinning shaft were chosen to be 56° C. and 53° C., respectively, and the height of the spinning shaft was adjusted to the same heights as chosen in Example 1.1. The temperature of the water bath was adjusted to 20° C. The hollow fibers were assembled in standardized mini-modules according to Example 2 for further tests.

1.18 Comparative Example: High-Flux Membrane 7

Membrane 7 (FIG. 2 ) refers to hollow fiber membranes which were extracted from an FDY-210GW filter (2.1 m² from Nikkiso Co., LTD.) which comprises a so-called PEPA® membrane (Polyester-Polymer Alloy, with PVP) having a wall thickness of 30 μm and an inner diameter of 210 μm. The dialyzer was developed for applications that require an extended sieving coefficient profile. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.19 Comparative Example: High-Flux Membrane 8

Membrane 8 (FIG. 2 ) refers to hollow fiber membranes which were extracted from an FDY-21GW filter (2.1 m² from Nikkiso Co., LTD.) which comprises a so-called PEPA® membrane (Polyester-Polymer Alloy) having a wall thickness of 30 μm and an inner diameter of 210 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.20 Comparative Example: High-Flux Membrane 9

Membrane 9 (FIG. 2 ) refers to hollow fiber membranes which were extracted from an FLX-21 GW filter (2.1 m² from Nikkiso Co., LTD., PVP-free) which comprises a so-called PEPA® membrane (Polyester-Polymer Alloy) having a wall thickness of 30 μm and an inner diameter of 210 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.21 Comparative Example: High-Flux Membrane 10

Membrane 10 (FIG. 2 ) refers to hollow fiber membranes which were extracted from a PES-21 SEα^(eco) hemodialyzer (Nipro, Japan). The hollow fiber membranes are polyethersulfone based membranes. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.22 Comparative Example: High-Flux Membrane 11

Membrane 11 (FIG. 2 ) refers to hollow fiber membranes as used in Polyflux® 170H filters (1.7 m², Gambro Lundia AB) which are based on a blend of polyarylethersulfone (PAES), polyvinylpyrrolidone (PVP) and polyamide and have a wall thickness of 50 μm and an inner diameter of 215 μm. The hollow fibers were assembled in standardized mini-modules according to Example 2 for further tests.

1.23 Comparative Example: High-Flux Membrane 12

Membrane 12 (FIG. 2 ) refers to hollow fiber membranes which were extracted from an EMiC®2 filter (1.8 m² from Fresenius Medical Care Deutschland GmbH). The respective hollow fibers are based on polysulfone and have a wall thickness of 35 μm and an inner diameter of 220 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.24 Comparative Example: High-Flux Membrane 13

Membrane 13 (FIG. 2 ) refers to hollow fiber membranes which were extracted from a PES-21 Sα^(eco) hemodialyzer (Nipro, Japan). The hollow fiber membranes are polyethersulfone based membranes. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.25 Comparative Example: Low-Flux Membrane a

Membrane a (FIG. 2 ) refers to hollow fiber membranes as used in Polyflux® 21L filters (2.1 m², Gambro Lundia AB) which are based on a blend of polyarylethersulfone (PAES), polyvinylpyrrolidone (PVP) and polyamide and have a wall thickness of 50 μm and an inner diameter of 215 μm. The hollow fibers were assembled in standardized mini-modules according to Example 2 for further tests.

1.26 Comparative Example: Low-Flux Membrane b

Membrane b (FIG. 2 ) refers to hollow fiber membranes which were extracted from an APS 21E hemodialyzer (2.1 m², Asahi Kasei Medical Co., Ltd.). The hollow fiber membranes are polysulfone based membranes with a wall thickness of 45 μm and an inner diameter of 200 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.27 Comparative Example: Low-Flux Membrane c

Membrane c (FIG. 2 ) refers to hollow fiber membranes which were extracted from an APS 21EL hemodialyzer (2.1 m², Asahi Kasei Medical Co., Ltd.). The hollow fiber membranes are polysulfone based membranes with a wall thickness of 45 μm and an inner diameter of 200 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

1.28 Comparative Example: Protein Leaking Membrane

The protein leaking membrane (FIG. 2 , (▾)) refers to hollow fiber membranes which were extracted from an Filtryzer BK-1.6F filter (1.6 m² from Toray Industries, Inc.) which comprises a so-called PMMA membrane (poly(methyl methacrylate)) having a wall thickness of 30 μm and an inner diameter of 210 μm. The hollow fibers were used for preparing standardized mini-modules according to Example 2 for further tests.

Example 2

Preparation of Filters, Hand Bundles and Mini-Modules

Filters can be prepared by introducing a fiber bundle into a dialyser housing. The bundle is potted with polyurethane, ends are cut, on both sides of the dialyser a header is fixed to the housing, the dialyser is rinsed with hot water and dried with air. During this last drying step, a certain amount of about between 10 g and 30 g of residual water per m² effective membrane area is left on the dialyser. After labelling and packaging, the dialyser can be steam-sterilized within the packaging in an autoclave at 121° C. for at least 21 min.

The preparation of a hand bundle after the spinning process is necessary to prepare the fiber bundle for following performance tests with mini-modules. The first process step is to cut the fiber bundles to a defined length of 23 cm. The next process step consists of melting the ends of the fibers. An optical control ensures that all fibers are well melted. Then, the ends of the fiber bundle are transferred into a potting cap. The potting cap is fixed mechanically and a potting tube is put over the potting caps. Then the fibers are potted with polyurethane. After the polyurethane has hardened, the potted membrane bundle is cut to a defined length and stored dry.

Mini-modules (fiber bundles in a housing) are prepared in a similar manner. The mini-modules ensure protection of the fibers and can be used for steam-sterilization. The manufacturing of the mini-modules comprises the following specific steps:

-   -   (A) The number of fibers required is calculated for a nominal         surface A of 360 cm² according to the following equation:         A=π×d _(i) ×l×n,     -   wherein d_(i) is the inner diameter of fiber [cm], n represents         the amount of fibers, and l represents the fiber length in the         housing (17 cm).     -   (B) The fiber bundle is cut to a defined length.     -   (C) The fiber bundle is transferred into the housing before the         melting process.

Example 3

Dextran Sieving Measurements

3.1 Dextran Solutions

Fractions of dextran supplied by Fluka (Mw 6, 15-20, 40, 70, 100, 200, 500 kDa) and Sigma-Aldrich (Mw 9-11 kDa) (both from Sigma-Aldrich Co. LLC, St. Louis, USA) were used without further purification. Solutions of dextrans with the different molecular weight fractions were combined in Millipore water (i.e., Type 1 ultrapure water, as defined by ISO 3696) at a concentration of 1 g/l for each fraction, which results in an overall concentration of 8 g/l.

3.2 Devices and Sample Preparation

For characterizing the membranes according to the invention and comparing them with membranes known from the prior art, it was necessary to eliminate the differences between devices caused by having different membrane surface areas or fiber numbers. Therefore, standardized mini-modules with a surface area of from 280 cm² to 300 cm² were manufactured from the membranes according to the invention or from membranes according to the prior art. In cases where the prior art membranes were part of complete filter devices, the membrane was extracted from said devices and mini-modules were prepared therefrom. Each mini-module had a nominal length of 170 mm, an effective length of approx. 120 mm to 150 mm (without PU potting) and an inner diameter of 10 mm. The internal diameter of fibers ranged between 170 μm and 220 μm, and the wall thickness between 30 μm and 50 μm (depending on the specific membranes used, see Examples 1.1-1.28 for details). Hence, the packing density also varied between 23% and 31%. All mini-modules were immersed in water for 30 min before the filtration experiments. Mini-modules to be characterized after contact with blood first have to be perfused with blood (bovine, 32% of hematocrits, 60 g/l of protein content and 1600 units/1 of heparin) for 40 min and rinsed afterwards with water for 30 to 60 min, as proposed elsewhere (Kunas G A, Burke R A, Brierton M A, Ofsthun N J. The effect of blood contact and reuse on the transport properties of high-flux dialysis membranes. ASAIO J. 1996; 42(4):288-294).

3.3 Dextran Sieving Coefficient Tests

Filtration experiments were carried out under a constant shear rate (γ=750 s⁻¹) and with the ultrafiltration rate set at 20% of the blood side entrance flux Q_(Bin), calculated as:

$Q_{Bin} = \frac{\gamma \cdot n \cdot \pi \cdot d_{i}^{3} \cdot 60}{32}$ where Q_(Bin) is the flux at the blood side entrance in ml/min; n is the number of fibers in the minimodule; d_(i) is the inner diameter of the fibers in cm and γ is the constant shear rate mentioned above. A scheme of the experimental setup is shown in FIG. 3 . As can be seen, the filtration conditions are without backfiltration, contrary to the conditions typical of hemodialysis. Additionally, the chosen conditions assure a filtration regime since the Peclet-number for all the investigated membranes is well above 3 even for molecules in the 0.1 kDa to 1 kDa range. The dextran solution was recirculated at 37° C.±1° C. Feed (blood side entrance), retentate (blood side exit), and filtrate (dialysate exit) samples were taken after 15 min. Relative concentration and molecular weight of the samples were analyzed via gel permeation chromatography. The analysis was carried out in a High Performance Liquid Chromatography (HPLC) device (HP 1090A or Agilent 1200; Agilent, Santa Clara, Calif., USA) equipped with an RI detector (G1362 from Agilent) and TSKgel columns (PWXL-Guard Column, G 3000 PWXL, G 4000 PWXL; Tosoh, Tessenderlo, Belgium). Samples were filtered through a 0.45 μm filter type OE67 from Schleicher and Schnell, Einbeck, Germany. Calibration was done against dextran standards (Fluka). The sieving coefficient SC is calculated according to the equation as follows:

${SC} = \frac{2 \cdot c_{F}}{c_{P} + c_{R}}$ where c_(F) is the concentration of the solute in the filtrate, c_(P) its concentration in the permeate and c_(R) its concentration in the retentate.

3.4 Results of the Dextran Sieving Coefficient Tests

TABLE III MWCO and MWRO values Average Membrane MWRO (90%) MWCO (10%) Membrane Classification MW [D] MW [D] Membrane A (Ex. 1.1) Invention 12.100 99.000 Membrane B (Ex. 1.2) Invention 11.300 81.000 Membrane C (Ex. 1.3) Invention 10.000 64.000 Membrane D (Ex. 1.4) Invention 11.600 88.000 Membrane E (Ex. 1.5) Invention 11.700 90.000 Membrane F (Ex. 1.6) Invention 11.921 105.000 Membrane G (Ex. 1.7) Invention 10.223 71.000 Comparative Example High cut-off 15.000 300.000 Membrane β (Ex. 1.8) Comparative Example High cut-off 19.300 200.000 Membrane α (Ex. 1.9) Comparative Example High cut-off 17.000 300.000 Membrane γ (Ex. 1.10) Comparative Example High cut-off 12.020 150.000 Membrane ϕ (Ex. 1.11) Comparative Example High-flux 9.700 50.500 Membrane 1 (Ex. 1.12) Comparative Example High-flux 6.600 33.000 Membrane 2 (Ex. 1.13) Comparative Example High-flux 7.300 67.000 Membrane 3 (Ex. 1.14) Comparative Example High-flux 5.800 28.000 Membrane 4 (Ex. 1.15) Comparative Example High-flux 4.400 18.900 Membrane 5 (Ex. 1.16) Comparative Example High-flux 5.300 43.000 Membrane 6 (Ex. 1.17) Comparative Example High-flux 8.300 30.000 Membrane 7 (Ex. 1.18) Comparative Example High-flux 7.000 32.600 Membrane 8 (Ex. 1.19) Comparative Example High-flux 5.800 58.000 Membrane 9 (Ex. 1.20) Comparative Example High-flux 8.300 45.000 Membrane 10 (Ex. 1.21) Comparative Example High-flux 5.900 50.000 Membrane 11 (Ex. 1.22) Comparative Example High-flux 7.300 40.000 Membrane 12 (Ex. 1.23) Comparative Example High-flux 8.700 58.000 Membrane 13 (Ex. 1.24) Comparative Example Low-flux 2.200 19.000 Membrane a (Ex. 1.25) Comparative Example Low-flux 3.060 13.000 Membrane b (Ex. 1.26) Comparative Example Low-flux 2.790 10.000 Membrane c (Ex. 1.27) Comparative Example Protein 3.000 67.000 Protein Leaking Leaking Membrane (Ex. 1.28)

Example 4

Albumin, β2-M and Myoglobin Sieving Coefficients

Middle molecules, consisting mostly of peptides and small proteins with molecular weights in the range of 500-60,000 Da, accumulate in renal failure and contribute to the uremic toxic state. Beta2-microglobulin (beta2-MG or β2-M) with a molecular weight of 11,000 is considered representative of these middle molecules. Myoglobin has a molecular weight (MW) of about 17 kDa is already larger and will not be cleared from blood to the same extend by known high-flux dialyzers, whereas it is readily removed by high cut-off dialyzers. Finally, albumin with a MW of about 67 kDa is a key element in describing the sieving characteristics of membranes, as albumin should not be allowed to pass a membrane for chronic hemodialysis to a significant extent.

The sieving coefficients for said proteins were determined for Membrane A according to the invention, Membrane 6, and for Membrane β according to EN1283 (Q_(B)max, UF=20%) in bovine plasma at with Q_(B)=600 ml/min and UF=120 ml/min. Further measurements were carried out at Q_(B)=400 ml/min and UF=25 ml/min according to DIN EN ISO8637:2014 (see Table IV). The bovine plasma used had a total protein concentration of 60±2 g/l. Myoglobin from horse heart (M1882) was purchased from Sigma-Aldrich Co. LLC. Purified β2-M (PHP135) was obtained from Bio-Rad AbD Serotec GmbH or Lee Bio Solutions (St Louis, Mo., U.S.A.) and diluted in bovine plasma. The resulting test solutions had the following final concentrations: albumin as contained in the bovine plasma, myoglobin (100 mg/l), β2-M (3 mg/l). The test solutions were gently stirred at 37±1° C. The respective mini-modules as described in Example 2 were primed with 0.9% NaCl solution. The setup for the test was according to DIN EN ISO8637:2014. The final protein concentration of the test solution was 60±5 g/l. Table IV summarizes the blood flow and ultrafiltration rates used and the average sieving coefficients obtained.

TABLE IV Sieving coefficients for albumin, β2-M and myoglobin Example/ Membrane Type Albumin β2-M Myoglobin (a) Comparative Example: High-Flux Membrane (Membrane 6) Ex. 1.17 Q_(B) = 600 ml/min; Q_(B) = 600 ml/min; Q_(B) = 600 ml/min; Membrane 6 UF = 120 ml/min UF = 120 ml/min UF = 120 ml/min <0.01 0.70 n.d. Q_(B) = 400 ml/min; Q_(B) = 400 ml/min; Q_(B) = 400 ml/min; UF = 25 ml/min UF = 25 ml/min UF = 25 ml/min <0.01 0.85 0.81 (b) Comparative Example: High Cut-Off Membrane (Membrane β) Ex. 1.8 Q_(B) = 600 ml/min; Q_(B) = 600 ml/min; Q_(B) = 600 ml/min; Membrane β UF = 120 ml/min UF = 120 ml/min UF = 120 ml/min 0.2 n.d. 0.95 Q_(B) = 400 ml/min; Q_(B) = 400 ml/min; Q_(B) = 400 ml/min; UF = 25 ml/min UF = 25 ml/min UF = 25 ml/min 0.44 >0.9 1.0 (c) Membranes according to the Invention (Membrane A, Membrane B, Membrane C) Q_(B) = 600 ml/min; Q_(B) = 600 ml/min; Q_(B) = 600 ml/min; UF = 120 ml/min UF = 120 ml/min UF = 120 ml/min Ex. 1.1 0.03 0.78 0.81 Membrane A Ex. 1.2 0.02 0.84 0.80 Membrane B Ex. 1.3 0.02 0.76 0.75 Membrane C Q_(B) = 400 ml/min; Q_(B) = 400 ml/min; Q_(B) = 400 ml/min; UF = 25 ml/min UF = 25 ml/min UF = 25 ml/min Ex. 1.1 0.06 >0.9 >0.9 Membrane A Ex. 1.2 0.08 >0.9 >0.9 Membrane B Ex. 1.3 n.d. n.d. n.d. Membrane C

Example 5

Determination of Albumin Loss in a Simulated Treatment

The simulated treatment is performed, for example, with a AK 200™ S dialysis machine. During the treatment samples of 1 ml are secured from the dialysate side of the system after 15, 30, 45, 60, 90, 120, 150, 180, 210 and 240 minutes and the albumin concentration in the samples in mg/l is determined (BSA, Bovine Serum Albumin). Albumin loss is calculated with the help of SigmaPlot software by establishing a regression curve of the type f(x)=y_(o)+ae^(−bx). The albumin loss can be calculated by integration of the regression curve, F_((x)) from 0 to 240 minutes, i.e. F(x)=bxy_(o)−ae^(−bx).

The simulated treatment is carried out as follows. A bag with 0.9% NaCl (500 ml) is connected to the dialysis monitor. The blood pump is started and the test filter is rinsed at Q_(B)=100 ml/min, Q_(D)=700 ml/min, UF=0.1 ml/min with the said sodium chloride solution. Afterwards, the dialyzer is filled by using the prescribed dialysate flow. The bovine blood (5000±50 ml) is provided in a container and placed in a water bath at 38±1° C. 5 ml of heparin are added in the beginning and then every hour. The blood is carefully stirred throughout the treatment. The test can be run in HD or HDF mode. Standard parameters are Q_(B)=400 ml/min, Q_(D)=500 ml/min, UF=10 ml/min. In case UF is >0 ml/min substitution fluid has to be used. Blood flow, dialysate flow and UF rate are started and samples are taken from the dialysate side at the respective times. Albumin concentration in the samples can be determined according to known methods. 

The invention claimed is:
 1. A semipermeable membrane comprising i) at least one hydrophobic polymer component and ii) at least one hydrophilic polymer component, wherein the membrane has a molecular retention onset (MWRO) of between 9.0 kDa and 14.0 kDa and a molecular weight cut-off (MWCO) of between 55 kDa and 130 kDa as determined by dextran sieving before blood contact of the membrane, wherein the membrane is configured for retention of albumin.
 2. The semipermeable membrane of claim 1, wherein the membrane is a hollow fiber membrane.
 3. The semipermeable membrane of claim 1, wherein the membrane is a flat sheet fiber membrane.
 4. The semipermeable membrane of claim 1, wherein the hydrophobic polymer component is poly(aryl)ethersulfone (PAES).
 5. The semipermeable membrane of claim 1, wherein the hydrophilic polymer component is polyvinylpyrrolidone (PVP).
 6. The semipermeable membrane of claim 1, wherein the hydrophobic polymer component is poly(aryl)ethersulfone (PAES) and the hydrophilic polymer component is polyvinylpyrrolidone (PVP), and wherein the membrane comprises between 2.0 wt.-% to 4.0 wt.-% PVP and a balance of poly(aryl)ethersulfone adding up to 100%.
 7. The semipermeable membrane of claim 4, wherein the membrane comprises a second hydrophobic polymer component.
 8. The semipermeable membrane of claim 7, wherein the second hydrophobic polymer component is polyamide.
 9. The semipermeable membrane of claim 2, wherein the hollow fiber membrane comprises a wall thickness from 30 μm to 49 μm.
 10. The semipermeable membrane of claim 2, wherein the hollow fiber membrane comprises a wall thickness from 30 μm to 40 μm.
 11. The semipermeable membrane of claim 2, wherein the hollow fiber membrane comprises an inner diameter from 170 μm to 200 μm.
 12. The semipermeable membrane of claim 2, wherein the hollow fiber membrane comprises an inner diameter from 175 μm to 185 μm.
 13. The semipermeable membrane of claim 1, wherein the membrane has a molecular retention onset (MWRO) of between 9.0 kDa and 12.5 kDa and a molecular weight cut-off (MWCO) of between 68 kDa and 110 kDa as determined by dextran sieving before blood contact of the membrane.
 14. The semipermeable membrane of claim 1, wherein the membrane has a molecular retention onset (MWRO) of between 10.0 kDa and 12.5 kDa and a molecular weight cut-off (MWCO) of between 68 kDa and 90 kDa as determined by dextran sieving before blood contact of the membrane.
 15. The semipermeable membrane of claim 1, wherein the membrane has a molecular retention onset (MWRO) of between 10.0 kDa and 12.5 kDa and a molecular weight cut-off (MWCO) of between 65 kDa and 90 kDa as determined by dextran sieving before blood contact of the membrane.
 16. The semipermeable membrane of claim 1, wherein the membrane has an average sieving coefficient for albumin of between 0.02 and 0.1.
 17. The semipermeable membrane of claim 1, wherein the membrane has an average sieving coefficient for albumin of between 0.02 and 0.08.
 18. The semipermeable membrane of claim 1, wherein the membrane has an average sieving coefficient for albumin of between 0.01 and 0.1.
 19. The semipermeable membrane of claim 1, wherein the membrane has an average sieving coefficient for albumin of between 0.01 and 0.06.
 20. The semipermeable membrane of claim 1, wherein the membrane comprises pores, and wherein the pores have an average radius, before blood or plasma contact, between 5.0 nm and 7.0 nm. 